1. Field of the Invention
The present invention relates to a laser module having one or more semiconductor laser elements, an optical fiber, and an optical injection system which couples one or more laser beams emitted from the semiconductor laser elements, to an end face of the optical fiber.
2. Description of the Related Art
Conventionally, the laser modules having the following construction are known as the pigtail-type laser modules in the field of optical communications. The pigtail-type laser modules are constituted by a semiconductor laser element contained in a package, an optical fiber being fixed to the package and having an end (light-entrance end) which appears inside the package, and an optical condensing system which couples a laser beam emitted from the semiconductor laser element, to a light-entrance end face of the optical fiber.
In the case where short-wavelength lasers in the blue or ultraviolet wavelength range or high-output-power lasers are used in the above laser modules, contaminants such as organic materials are likely to deposit on surfaces of optical components at which the optical power density is high, and cause troubles such as the lowering of the transmittance or breakage of the optical components. It is known that dust suspended in the air can deposit on incident surfaces of optical components (i.e., end faces of optical components on which laser beams are incident) by the dust collection effect, and products of reaction between the laser beams and organic materials existing around the end faces or vaporized in the air can also deposit on the end faces, as described in Japanese Unexamined Patent Publication No. 11(1999)-054852.
Japanese Unexamined Patent Publication Nos. 2004-252425 and 2004-253783 disclose techniques for preventing deposition of contaminants on end faces of optical components. According to the technique disclosed in Japanese Unexamined Patent Publication No. 2004-252425, a light-entrance end face of an optical fiber is held in close contact with an outer surface of a glass window of a package. In addition, according to the technique disclosed in Japanese Unexamined Patent Publication No. 2004-253783, a light-entrance end face of an optical fiber is protected by hermetically sealing the light-entrance end face or holding a transparent member (such as a glass block) in close contact with the light-entrance end face.
Further, Japanese Unexamined Patent Publication No. 2(1990)-081008 and U.S. Pat. No. 5,388,171 disclose laser modules which have similar constructions to the constructions disclosed in Japanese Unexamined Patent Publication Nos. 2004-252425 and 2004-253783, although the purposes of the techniques disclosed in Japanese Unexamined Patent Publication No. 2(1990) -081008 and U.S. Pat. No. 5,388,171 are different from the purposes of the techniques disclosed in Japanese Unexamined Patent Publication Nos. 2004-252425 and 2004-253783.
However, the techniques disclosed in the above patent publications have the following problems.
Japanese Unexamined Patent Publication No. 2004-252425 discloses a structure wherein a receptacle capable of receiving a ferrule is fixed outside a hermetically sealed package containing semiconductor lasers and lenses, and an optical fiber the tip of which is inserted into the ferrule is connected with the receptacle by using a connector which can fit into the receptacle. However, in this structure, the semiconductor lasers and the lenses are contained in the hermetically sealed package. Therefore, the size of the package increases. Generally, the hermetically sealed package is expensive, and the cost of the hermetically sealed package further increases when the hermetically sealed package is large. In addition, in the above structure, the ferrule is held in contact with the light-output window of the hermetically sealed package. Therefore, it is necessary that the glass window have a hermetically sealing function. In addition, since the glass window is pressed when the ferrule is kept in contact with the glass window, it is necessary to increase the strength of the glass and the sealing portion in order to maintain the hermetically sealing function against the pressure. However, the cost increases with the increase in the strength of the glass and the sealing portion.
On the other hand, as mentioned before, Japanese Unexamined Patent Publication No. 2004-253783 discloses a structure for preventing deposition of contaminants on the light-entrance end face of the optical fiber by fixing a transparent member to the light-entrance end face or holding the transparent member in close contact with the light-entrance end face. However, the transparent member is fixed to a chassis or a support, or to a tip of the optical fiber. Therefore, it is impossible to replace the transparent member. The constructions disclosed in Japanese Unexamined Patent Publication No. 2(1990)-081008 and U.S. Pat. No. 5,388,171 also have similar problems.
Further, if the constructions disclosed in the above patent publications are modified so that the optical fiber can be detached from and reattached to the glass window or the transparent member, minute dust or contaminants can be caught between the ferrule and the glass window or between the ferrule and the transparent member when connection and disconnection are repeated, so that the physical contact between the ferrule and the glass window or between the ferrule and the transparent member becomes insufficient, and the light-entrance end face of the optical fiber can be contaminated.